First, the tax preparer should be able to provide you with the adjusted cost basis of the property.
When real property is placed in service, the cost is typically split between the building and land. The cost allocated to the building is depreciable, the land is not.
Having said that, given the timing of when the property was first rented, its likely that the main building structure was depreciated using the accelerated cost recovery system (ACRS) method over either 15 years or 18 years. Thus, the main building structure is fully depreciated. However, the improvements start depreciating once they are made. They may also have been depreciated using either the ACRS method or the MACRS method which was mandatory for property placed in service starting in 1987. Also, in calculating your adjusted cost basis for gain calculation purposes, you must reduce your original cost basis by allowable depreciation whether it was actually deducted or not.
The botXXXXX XXXXXne is that I can not figure the allowable depreciation for you without knowing the exact original cost of the property, how it was split between land and building or how much improvements cost and when they were placed in service. Also, there may be other costs to consider such as refinancing costs that may be amortized and other property such as appliances and furniture that may have been capitalized and depreciated to some extent.
Get with the tax preparer and see what records they have available regarding the rental property tax basis. That is your best bet. Also, if your parents have had any rental losses suspended because they had too much income, those losses become deductible in the year of sale.